Silicon nitride sintered bodies are raw materials having excellent thermal properties and mechanical properties, and heretofore they have been introduced in different patent publications and literatures. In particular, a large number of silicon nitride sintered bodies have been introduced which can improve mechanical properties by using sialon as a principal phase.
Patent literature 1 and 2 propose silicon nitride sintered bodies with improved mechanical properties. That is, in the ternary composition chart of Si3N4— first assistant—second assistant, the first assistant is a combination of two kinds of Y2O3 and MgO, and the crystal phase in the obtained sintered body contains both of α-Si3N4 and β′-sialon. The β′-sialon in the sintered body is of a general formula Si6−ZAlZOZN8−Z (where 0≦Z≦1.0), enabling control of the crystal composition.
Patent literature 3 proposes, from the viewpoint that it is important to optimize the dissolved amount of Al and O into crystal lattices, a range required in a chart, on which the ordinate represents the weight ratio of one type or two or more types of rare earth metals and lanthanides metal oxides and one type or two or more types of aluminium oxides and aluminium nitrides, each being added as sintered additives, and the abscissa represents the ratio of a measured z value indicating the amount of substitutional solid solution of Al and O elements in β′-silicon aluminium oxynitride [β′—Si6−ZAlZOZN8−Z (0≦Z≦4.2)] crystal phase that can be measured by the X-ray diffraction of a sintered body, and an ideal z value to be calculated on the assumption that all of the Al elements of one type or two types of the added aluminium oxides and aluminium nitrides have been substituted and melt in solid state in the β′-silicon aluminium oxynitride crystal phase. This patent publication describes that, in the above-mentioned range, the shape of a fibrous Si3N4 crystal grain can be controlled, enabling improvements in the strength and the toughness of a Si3N4 sintered body. There is also proposed its manufacturing method of heat treating a Si3N4 raw material powder, in which 60% or more of oxygen impurities contained in the powder obtained by imide pyrolysis are present in the surface part of the raw material powder, in an atmosphere of N2 gas at 1350 to 1650° C. for two hours or more.
Patent literature 4 proposes a high toughness sialon sintered body composed of sialon particles and grain boundary phases. The total amount of the grain boundary phases is 20 wt % or less. In a three-component system of SiO2—Al2O3—Y2O3Si, Al, and Y of the grain boundary phases in terms of oxide, the (SiO2, Al2O3, Y2O3) compositions in weight ratio are within an area surrounded by four points of a point A (20, 10, 70), a point B (20, 25, 65), a point C (30, 25, 55), and a point D (30, 10, 60). That is, this publication describes that the composition and the total amount of the grain boundary phases are important for high toughness.
Patent literature 5 proposes a silicon nitride composite sintered body containing Y and Al elements. That is, 0.5 to 5 volume % of different type particles, which are the oxides or the nitrides of Groups IVa and Va in the periodic table, and have a coefficient of thermal expansion of 5×10−6/° C. or above and a mean particle diameter of 1 to 500 nm, are dispersed in silicon nitride and/or sialon crystal grains and grain boundary phases, which have a mean diameter of shorter axis of 0.05 to 3 μm, and an aspect ratio of 10 or less. Dispersion of the nanometer-size different types of particles causes residual stress in the crystal grains and the grain boundary phases, thereby achieving 140 kg/mm2 or more in the four-point flexural strength thereof.    Patent literature 1: Japanese Patent Publications No. 2539961    Patent literature 2: Japanese Patent Publications No. 2597774    Patent literature 3: Japanese Patent Publication No. 2539960    Patent literature 4: Japanese Patent Publication No. 2820846    Patent literature 5: Japanese Patent Publication No. 2776471